The present invention relates to a noise suppressing circuit for suppressing pulse noise and multipath noise in an FM tuner.
In order to avoid radio disturbances in the FM tuner caused by various types of noise such as pulse noise and multipath noise, there are available various systems that have been incorporated into FM tuners.
FIG. 3 shows a conventional noise suppressing circuit. A composite signal obtained through a front end, an intermediate frequency amplifier and a detector (not shown) of the FM tuner is applied to a delay circuit 1 for delaying the signal. The delayed signal is applied to a gate 3 through a buffer amplifier 2. At the same time the composite signal is applied to a level detector 5 through a high-pass filter 4. The level detector 5 detects noise so as to produce output signals for closing the gate 3. The composite signal passes through the opened gate 3 and is charged in a capacitor C.
FIG. 4 shows the voltage at the capacitor C. If the noise N is included in the composite signal, the gate 3 is closed, so that the capacitor C holds the voltage during a period T corresponding to the overlapping period of the noise N as shown in FIGS. 5a and 5b. Thus, the composite signal is cut off for the period T.
The composite signal from the gate 3 is applied to a stereo demodulator 7 through a buffer amplifier 6. The composite signal is demodulated by the stereo demodulator 7 based on a pilot signal in order to provide an audio signal having right (R) and left (L) signals.
Thus, noise is eliminated, so that sound having good quality is obtained.
However, if the closing time of the gate 3 becomes long, a sub signal included in the composite signal is reduced, which causes the demodulated signal to distort. In the conventional circuit, therefore, the closing time of the gate 3 is set at 10 .mu.sec to 20 .mu.sec by a timer provided in a gate control circuit (not shown). However, the noise can not be completely eliminated.